The work proposed here seeks to investigate the mechanism by which mammalian macrophages suppress the metabolic checkpoint kinase Mechanistic target of rapamycin (MTOR) to promote immune defenses and counteract anabolic reprogramming by the vacuolar pathogen Legionella pneumophila. MTOR is a universally conserved master switch regulating catabolism to anabolism transition in eukaryotic cells. Vacuolar pathogens rely on host metabolites not only for bacterial replication but also for maintenance of their intracellular niches. The human respiratory pathogen Legionella pneumophila is a prototypical vacuolar pathogen that infects and replicates within mammalian macrophages. We uncovered that Legionella stimulates host membrane biosynthesis to expand its niche by subverting MTOR. However, a host suppression pathway counteracts L. pneumophila-induced MTOR activation by targeting MTOR for ubiquitin-dependent proteolysis. In macrophages, MTOR suppression promotes cell intrinsic (autophagy) and cell extrinsic (inflammation) host defenses. Specifically, pathogenic L. pneumophila expressing the type IVb secretion system (T4bSS) but not the avirulent mutants lacking a functional T4bSS elicited the host MTOR suppression response. Thus, we hypothesize that in mammalian macrophages MTOR functions downstream of homeostatic mechanisms that sense the pathogenic potential of invading microbes to potentiate host defenses. Such mechanisms operate by detecting virulence adaptations encoded by bacterial pathogens (such as specialized secretion systems and secreted toxins), which frequently produce pathogen-specific signatures. In the first aim we will determine the mechanism by which Legionella subverts MTOR function, when the host suppression pathway is inactivated. In the second aim we are going to use biochemical and genetic approaches to determine how the Legionella intracellular niche is controlled by host metabolic regulators. Our third aim is to elucidate how the pathogen signature that elicits MTOR suppression is generated. To this end, we will perform a systematic analysis of the Legionella intracellular niche using a variety of imaging approaches to define the early events that trigger MTOR suppression.